[Proj] OSTN02 question: GRS80 instead of Airy.

[Roger Oberholtzer]

On Mon, 2006-10-30 at 15:04 -0500, Frank Warmerdam wrote:
> Roger Oberholtzer wrote:
> > No takers on this one?
> > 
> > On Wed, 2006-10-25 at 12:58 +0200, Roger Oberholtzer wrote:
> >> I use the C API to do transformations. I am converting from GPS values
> >> in WGS84 to OSTN02 for the United Kingdom. My current code is:
> >>
> >>        fromProj = pj_init_plus(
> >>                 "+proj=latlong "
> >>                 "+ellps=WGS84 "
> >>                 "+towgs84=0,0,0 "
> >>                 "+nodefs")
> >>
> >>         toProj   = pj_init_plus(
> >>                 "+proj=tmerc "
> >>                 "+lat_0=49 "
> >>                 "+lon_0=-2 "
> >>                 "+k=0.9996012717 "
> >>                 "+x_0=400000 "
> >>                 "+y_0=-100000 "
> >>                 "+ellps=airy "
> >>                 "+nadgrids=gb.gsb "
> >>                 "+units=m "
> >>                 "+no_defs")
> >>
> >> Note that I do a grid shift, using data provided by a kind member of
> >> this list.
> >>
> >> My question is, how can I use the GRS80 ellipsoid instead of Airy? 
> 
> Roger,
> 
> Well, you would just change +ellps=airy to +ellps=GRS80.
> 
> But perhaps your question was more subtle?

Or not. If I change the +ellps=airy to +ellps=grs80, leaving all else
the same, I see no difference at all.

Ad an example, for lat=50.34180000 lng=-4.82595000, I get
Easting=199020.37000 Northing=52918.58000 for both airy and grs80.

It could be that these should be the same. But I thought grs80 would
result in some small difference.

--
Roger Oberholtzer